Validation rules for delivery service regex

The following rules need to be enforced in the deliveryservice regex apis:

1. POST /api/../deliveryservices/:dsId/regexes (create ds regex)

• you can't create a ds regex at position 0 because it was already created for you when you created the ds and there can only be 1 regex at position 0.

2. PUT /api/../deliveryservices/:dsId/regexes/:id (update ds regex)

• can't edit a ds regex to have set_number=0 (there can only be one and it already exists)
• can't update the set_number of the regex at position zero.
• the format of the ds regex at position zero must be validated to .*\something\..*
• if the pattern of the regex at position zero changes, then dnssec keys must be recreated if the ds is part of a dnssecenabled cdn

3. DELETE /api/../deliveryservices/:dsId/regexes/:id (delete ds regex)

• can't delete the regex at position zero